function [X_fit,Y_fit,a,b] = get_line(X,Y,index)
    X=[ones(length(index),1),X(index)'];
    Y=Y(index)';
    X_fit=X;
    coe=(X'*X)\(X'*Y);
    a=coe(1);
    b=coe(2);
    Y_fit=X*coe;
end